1. Field Of The Invention
The present invention relates to improved high heat acrylics of the type described in the '615 patent (bulk polymerization process using multiple initiators, multiple time-temperature cycles including raising the polymerization temperature above the glass transition temperature of the final product. Specifically, the present invention is directed to miscible blends of these high heat resistant acrylate-maleimide polymers with conventional poly(methyl methacrylate) (PMMA) and compatible blends of the high heat resistant acrylatemaleimide polymers with acrylic impact modifier alone or, additionally, with conventional PMMA.
Within the term "conventional PMMA" is included not only homopolymers of methyl methacrylate, but also copolymers of methyl methacrylate together with minor amounts of copolymerizable monomers which are present to an extent below that which results in a significant reduction in optical properties in comparison with a homopolymer of methyl methacrylate. In general, when the conventional PMMA is a copolymer it should be formed from a copolymerizable mixture of methyl methacrylate with not more than 15% by weight of a comonomer based on the weight of the copolymerizable mixture and the comonomer should preferably be selected from alkyl acrylates or other alkyl methacrylates.
2. Description Of The Related Art
In general, acrylic polymers are well-known and are commercially desirable for their excellent optical properties, weatherability properties and cost for numerous end uses in various industries such as automotive, electronics, telecommunications, lighting, optics, business machines and decorative products for specific products such as lighting fixtures, automobile light lenses, dials, video discs, opthalmic lenses and other articles where durable, weatherable, clear features are desired.
In order to obtain specific, optimized physical properties of acrylic polymers, it is also well-known to polymerize different monomers to form co- or terpolymers. Copolymerization of a mixture of two or more monomers often led to polymers which had different and/or more suitable physical properties than either of the homo-polymers alone, or mechanical blends of polymers formed from the individual monomers.
The most popular commercial acrylate polymer in the acrylic industry at present is PMMA, which has relatively high clarity and excellent light transmission properties. Furthermore, PMMA has excellent resistance to sunlight, low density, durability and is pleasing in appearance. However, PMMA has a relatively low heat distortion temperature (HDT) and relatively low glass transition temperature (sometimes referred to as Tg). Conventional PMMA is satisfactory for most uses only for relatively short times at temperatures between 90.degree. C. and 102.degree. C., and the glass transition temperature of PMMA copolymers generally falls within the range of 85.degree.-110.degree. C. Thus, for many uses, especially in lighting systems, the HDT's and Tg's of conventional PMMA are unsatisfactorily low.
The '615 patent discloses process for making and high temperature, heat resistant acrylate-maleimide polymers which have transparency properties about the same as the transparency properties of conventional PMMA and are weather-resistant, but have glass transition temperatures from about 105.degree. C. to 131.degree. C., which temperatures are significantly higher than for conventional PMMA. However, the acrylate-maleimide polymers of the '615 patent are relatively brittle and relatively expensive compared to conventional PMMA-based polymers.
It has been found that a need exists for clear, PMMA-like polymers having Tg's between the Tg of PMMA and the Tg's of the polymers of the type disclosed in the '615 patent. Although mechanical blending of polymer is well-known, the physical properties and behavior of mechanical blends are often unpredictable. For example, a mechanical blend of two clear polymers might be opaque and thus not useful for applications in which transparency is required.
It is also known that a high softening point acrylic terpolymer known as "Diakon" NS 9300, formerly marketed by Imperial Chemical Industries, was incompatible with conventional PMMA. This polymer, being a terpolymer formed from a mixture of 85% by weight methyl methacrylate, 10% by weight N-o-chlorophenylmaleimide and 5% by weight of styrene, was prepared by a suspension polymerization process according to the general disclosure of U.S. Pat. No. 3,676,404. It was observed to be incompatible with conventional PMMA because when conventional PMMA was used to purge, that is clean out, an injection molding machine previously used in molding "Diakon" NS 9300, the purged material, that is a mixture of conventional PMMA and "Diakon" NS 9300, was milky white and hazy in appearance, whereas the individual materials were clear.